update README.md to document nfs-experimental plan
update README.md to document nfs-experimental plan

[#154075645](https://www.pivotaltracker.com/story/show/#154075645)

Signed-off-by: Kai Xiang <987005552a526059a0c9b4b70d166beabef8374e@pivotal.io>

cloudfoundry-incubator/nfs-volume-release,cloudfoundry-incubator/nfs-volume-release,cloudfoundry-incubator/nfs-volume-release